1. Technical Field of the Invention
The present invention relates to a method and a transceiver for a data packet oriented communication system. Even more particularly the present invention relates to a method and a transceiver for selecting a current transmission mode from a set of available transmission modes, where each transmission mode provides a specific transmission rate for transmitting the data packets.
2. Description of Related Art
A data packet oriented communication system, where at least two transceivers are connected by a radio link is a part of a data packet oriented communication network, which includes several transceivers connected by several physical links. A physical link, where the data packets are transmitted via radio transmission is called a radio link. Examples for such a data packet oriented communication system with a radio link are standardised Hiperlan/2 systems, IS-136 systems or EDGE systems.
In general a data packet oriented communication system can be described with the help of a Reference Model, which is developed by the International Standards Organisation [Tanenbaum, Andrew S.: “Computer Networks”, 3rd ed., Prentice-Hall International Inc. 1996, p.28ff]. Here, each transceiver is depicted as having several layers. Each layer represents an exact defined system function of the transceiver and can be developed independent from each other.
Such a Reference Model is for example the TCP/IP Reference Model, which can be used to describe a data packet oriented communication system with a radio link, like e.g. a HIPERLAN/2 system. Here, the layer closest to the radio link, is the Host-To-Network-Layer. This Host-To-Network-Layer can be subdivided into the Physical-Layer (PHY-Layer) and the Data-Link-Layer (DLC-Layer). Both layers perform efficient and reliable data transmission via the radio link. Above the DLC-Layer there is the Internet-Protocol-Layer (IP-Layer). In order to guarantee a reliable data transmission between a source transceiver and a destination transceiver and to avoid congestion in the network, a Transport Control Protocol Layer (TCP-Layer) is used above the IP-Layer. Next to the TCP-Layer there is the Application-Layer, which represents the different applications, like Internet applications etc.
Data packets created on an Application-Layer are transferred through the subsequent layers, wherein on each layer the data packets are temporary stored in a buffer, before transferred to the next layer. Finally on an Physical-Layer a transmission mode is selected for transmitting the data packets on the radio link. The data packets are transmitted on the radio link in time frames. In general, the performance of each layer and the radio link is measured in a throughput or transmission rate value. The throughput as well as the transmission rate are defined as the ratio of the amount of data and the time needed for the transmission of the data. Both, the throughput and the transmission rate have an unit of bit per second (bit/s) and can be measured on each layer of the data packet oriented communication system. The throughput on the TCP-layer is the most important one, because this further called as the overall throughput is visible to an user of that data packet oriented communication system.
As described in [Jamshid Khun Jush: “Structure and performance of the Hiperlan/2 Physical Layer”; Procedures VTC′99 Fall 1999], the key feature of the Physical-Layer is to provide several transmission modes with different coding and modulation schemes, which are selected by link adaptation. Depending on the radio link quality, the PHY-Layer selects for each link quality parameter, that transmission mode with the highest transmission rate. As a result, on the PHY-Layer the throughput of data packets is optimised depending on the selected transmission mode.
Such a method for selecting a transmission mode is also known from WO99/12304. For it, the transmission rate for all available transmission modes will be compared. The transmission mode with the maximum transmission rate is selected as the suitable transmission mode for transmitting the data packets via the physical link.
Also as e.g. known from U.S. Pat. No. 5,526,399, in conventional mobile radio communications systems a method for realising error free data transmission is employed. Such a method is the automatic repeat request (ARQ) mechanism, wherein a request for transmission of data is repeated at the transmission side, when a transmission error of data occurs on the reception side. This method is also applied in data packet communication systems. Therefore on the layer at least parts of the buffer size are used as an automatic request buffer (ARQ-buffer) for temporarily storing data packets to be transmitted and data packets to be retransmitted, when erroneously transmitted before. Thereupon a state of the automatic request buffer in a layer depends on the erroneous transmitted data packets. The amount of erroneous transmitted data packets can be measured with an error rate value. The higher the error rate, the more of the buffer space is blocked by data packets to be retransmitted.
In EP00105836.1, a further method for selecting a transmission mode is shown. Here the selection, which is established on the PHY-Layer, is based on the radio link quality and the state of the automatic request buffer on the DLC layer. It is shown that on the DLC-Layer the throughput of data packets is improved, by taking into account the link quality and the buffer state on the DLC-Layer.
To achieve an optimal data throughput on the PHY-layer, the selection method according to WO99/12304 seems appropriate for data packet oriented communication systems, where the transceivers have very large ARQ buffers on the DLC- and TCP-layer. Under this assumption, the optimisation of the throughput on the PHY-layer and on the TCP-layer is equivalent. But this is an assumption that is not applicable for all data packet oriented communication systems. Contrary the selection method according to EP00105836.1 seems appropriate in applications made, where the buffer sizes on the DLC-layer is limited to about 30 kbyte and the buffer size on the TCP-layer is limited to 64 kbyte, like for example when personal computers or desktop computers are connected via a radio link.
Both above described methods are applicable under the assumption that the automatic request buffer on the TCP-Layer has a size that is larger than 30 kbyte. Then the overall throughput, which in the course of the following is understood as the data throughput between the TCP layer and the application layers in destination or receiving transceiver, can be optimised. FIG. 5a shows an simulation of such an overall throughput for an Hiperlan/2 system. Here the throughput for each selected transmission mode (M1, . . . ,M5) is depending from the link quality C/I and the dashed line (M0) shows the achievable optimised overall throughput.
But, if e.g. a Notebook will be used in such a data packet oriented communication system, then the buffer size on the TCP layer in such a notebook usually is limited to around 8Kbyte. If the link quality is poor, that means that the radio link quality is not error free, more data packets have to retransmitted and thus the ARQ buffer in the DLC layer of the source transceiver is filled. As a consequence also the ARQ buffer in the higher layers will be blocked. As a result the overall throughput in the destination transceiver, which is visible to an user, will be reduced. Additional, if the TCP buffer is limited, that TCP buffer will be blocked even faster and the overall throughput will decrease more. The conclusion is that a limited buffer is more sensitive to the radio channel error than a large buffer. This then leads to the disadvantage that the maximum throughput will not be achieved. In FIG. 5b the dashed line shows the simulated overall throughput M0′ for a Hiperlan/2 system, where the buffer size on the TCP layer is limited and where a transmission mode M1′, . . . ,M5′ is selected on the PHY-layer, without regarding the limitation of the buffer size on the TCP layer. Thus, while the DLC-Layer, selects the current transmission mode without any knowledge about the buffer size in the TCP-Layer, the overall throughput is not optimised. More specifically, if the lower layer selects a transmission mode under the assumption of an unlimited buffer size in the higher layer, but that buffer size is limited, then the overall throughput decreases. This restricts the system performance not only in the above described applications, but also in applications, where for example a combination of different transceivers like a personal computer with an unlimited TCP buffer and a notebook with a limited buffer are connectable via a radio link.
A conceivable solution to overcome the above described problems could be, that the different layers provides each other with a protocol about their current state, like e.g. the fullness or state of their buffer. But this seems not practical, because then the layers which are usually developed independent from each other have to be modified. Therefore the standard for the data packet oriented communication system has to be modified, which needs a long time and which is normally very expensive, because the existing transceivers must be additionally equipped with the new standards. More precisely new protocols between the layers have to be developed and standardised.
Consequently, a need exists for a method and a transceiver in a data packet oriented communication system, which avoids the described above disadvantages. It is therefore a principle object of the invention to have a method and a transceiver, where a current transmission mode is selected on the lower layer, which achieves a maximum and robust transmission rate and therefore an optimised overall throughput, which is visible to an user. It is still further an object of the present invention to have a method and transceiver, where a maximum and robust transmission rate is achieved, which is independent of the buffer size on the higher layers. Further objects of the present invention will become clear as the description proceeds.